Observation instrument



2 sheets-sheet 1 T. L. THURLOW OBSERVATION INSTRUMENT Filed.v April ll, 1944 sept. 11, 194,5.

Sept. ll, 1945. v1'. L. THURLow OBSERVATION INSTRUMENT Filed April 11, 1944 2 Sheets-Sheet 2 Patented Sept. 11, 1945 oBsERva'rIoN msfmlmiinr Thomas L. Thurlowpveniee, Calif., f Appucafmnnprir 1r, 1944,;*serial- No. 530565 25 claims. (crees-2,14)

(Granted under die' act of Mareas; 188s, as

amended April 30, 1928;v 37010; G2 7.57)` f The.I invention described herein may be manul factured and used by or for thel Government for governmental purposes, without. thev payment. to me of .any royalty thereon.

This invention relates to a device for facilitating tire taking of accurate readings froman observation instrument, and more particularly, .to a device for limiting the rapidity withwhich; the adjusting screw or knob of the instrument may be manipulated forv the purpose of obtaining;v a readingtherefrom.

Reference is made tomy copending application Serial No. 530,566 led. April 11, 1944, and entitled Horizontal reference means for sextants: and the like, which claims certain features of the invention. herein. shown. and. described but which are not encompassed within the. scope of the claims appended hereto;

Inr the earlier forms ofr sextantst it. was customary to use the earths-` horizon asa reference from which to measure the altitude of the various celestial bodies. This 'method was not entirely satisfactory, however, since under unfavorable weather conditions it was impossible. for the observer to see the horizon. This type of sextant was also unsuited for usein. aircraft since an error was introduced into the sextant reading due to the elevation of the aircraft. abovetheV earths surface. In order to overcome this difficulty a new type of sextant was devised which incorporated an articial horizon within the.y instrument itself, thereby eliminating. the needfor sighting on the actual horizon of the earth. 'I'he articial horizon, however, wassubject to accel-l eration errors and it therefore became necessary to provide means for minimizing this error. LAC cordingly, various averaging and integrating structures have been designed in an attempt to. reduce the acceleration error to a tolerable gure. While the structures so designed have been successful in materially reducing the errors intro'. duced into the reading on account of the eiect of acceleration on the artificial horizon, at the same time they have slowed down the operationV of taking a reading and have greatly increased the cost ofthe instrument.

Accordingly it is an object of` the present in-` vention to provide a simple and expeditious means for obtaining an accurate reading from an observation instrument in which either the ob i ject being observed or the point of reference is constantly moving abouty a medial point.

Another object of the invention is izo-provide"- a simple and expeditious means for reducing; the error caused in the reading of anarticiat horizon type` of7 sexta-ntby reason of the eiect of. acceleration. upon the4 horizontalv reference. A ,further` object of the invention. is to. provide a. means; for.' limiting.: the rapidity. withy whichthe ndex .mirroir of. an: artificial horizon. type' of sextant may bemanipulat'ed;

. Still: ai furtherv object' of the" invention is to provide. escapement brakey on an. artiiieial` horizonfltype.- off sextant for limiting. the speed withr which the; indexv mirror may' be` rotated;

\ lend thereof.

. In the drawings; Figi. 11 is a sectional side elevation of a sextant incorporating the present invention. l y

Fig, 2" isf a. sectional front elevation of a. prir-t tion.o-thesextantshownIinfFig. 1i 1 Fig.. 3 is a. fragmentary plan view of the.` sextant shown inlFigf, lwith certainparts; removedf solas. tumore.-clearfly;A show the escapeinent type of brake used in the` present embodimentof the inventioni v As shownin l, the sextant. comprises a frame Il)l` which serves to support and; protect the various component parts of. the; instrument. ThisY frame Aconstitutes the right side,l bottom) andback oi the instrument, the left-hand' side beingenclesed by a cover plate: Il (see Fig. 2) which is secured toythe'f-rame I-U by screws I3. The `front andY top of theinstrument are leftfuncovered` sot as to.A leave: the,` index mirror exposed: tov coming from the front or` from overhead. Suitable handgrips' (not shown).- are secured to the side of. the frame Hl and tofthe: side: ofr the cover plate tif so as to enable the observer to hold the instrumentwithbeth hands while: making an observation. An index` mirror i2: issecuref to a rotatable supporting frame I4v by. means vof spring clips we The? position of the` mirror onv the sup-` parting,y frame may be adjusted with a: high degree jot precision by means of the screws' t8 which arethreaded into theY frameaM and' bear againstr tlieundgerside: of the mirror l2f. Thev framev l lt. is provided withl a: stub shaft 2l), which is journaled suitable. bearings located inthe sextant frame.

tot 'llhe indexfmirror is. rotatable onv the stub i shaft 2uby means of aworm ,22: which engages a gear. sector 24. secured'. toi an arm.A 26 which is formed. integrally with themirror supporti-ng frame t4., f

A. horizon glass 218, which; iny this 1 instance conof a: completely siivered' mirror, isk securedf an objective glass 44, after which it emerges from the instrument through an aperturesprovided'i in a rubber-cushioned eyepiece 48V which is secured to the frame l0. Between-^the horizon 4 glass 28 and the aperture 40 is- @disk 50whih is rotatably mounted on a shouldered screw 52 secured to the rear wall of the frame 10t- The disk is provided with a, series .of i apertures 53,1A one of which is left open and the ,remainder` of which are covered by filter glasses 54 of different shades and densities for viewing the various types of celestial bodies. By rotating the disk 50 on the screw 52 any one of the various apertures 52 may be brought'into positionrin front ofthe aperture. 40v so as. tosuitably'lter 'the ray=of= light emanating from the celestial body;

The present sextant makes use of a novel type of artificial horizon which formsthe subject mat` ter of my aforementioned copending application. Since the present invention is not limited to use with the particular Vtype of artificial horizon shown herein, but may be `used withany type of artificial horizonwhich' is subject to acceleration errors, it will be sufllcient for theA present purposev to describe only as much of this part of the struc-L ture as is necessary for a complete understanding of the invention. f l Y Secured to the frame I at the rear ofthe instrument is a collimator tube 56 which is closed at the upper end by a thin metal diaphragm 58. The diaphragm 58 is,` suitably pierced to provide two sh'ort parallel slits 60 therein through which light from above the instrument may enter'th collimator tube 56. The light passing through the slits is collimated by the lens B2 after which it passes through the inclined glassplate 42 and proceeds on downwardly toward Vthe bottom of the collimator tube. On therlower end of th'e tube 55 is secured a liquid cell 64, the bottom 'vof which is closed by a circular mirror 66 heldin place by a closure plug 68. The upper Vsurface of the mirror is covered to a depth of approxi-g mately five millimeterswith a clear colorless lid; uid having an index of refractionof 1.5. The liquid is prevented from running out ofthe cell 64 when the sextant is tilted from side to sidek 0r turned upside down by means of a trap 'I2 locatedv on the inside of the cell. 'The collimated light from the slits 60 passes downwardly through the liquid 10 and strikes the silvered surface of the' mirror 66, after which it is reflected back up through the liquid and into the collimating tube 56. The ray of light then strikes the under side of the inclined glass plate 42 `and is reilected out of the tube 56 through the objective glass 44 and the eyepiece aperture 46. The liquid'- 10 contained in the bottom ofthe vcell 64 causes the rays of light from the slits 60 to emerge from'the eyepiece 48 in a horizontal direction even though the sextant be tilted slightly forward or backward from the vertical. '.This is due tothe fact that the liquid assumes a wedge-shaped cross-section -y coming ray by just the proper amount to maintain the emergent ray horizontal. In this way, the image of the slits will always tend to remain in coincidence with the image of the celestial body even though the sextant be inadvertentl tilted by the observer.

The sextant herein shownr and described is primarily intended for use on board aircraft and will therefore be subject to all of the accelerations to which the aircraft itself is subjected. Such accelerations will cause the liquid 'l0 to pile upon one side or the other of the cell 64 and thus cause an unwanted deviation of the artificial horizon'image'.` In other words, if the observer has the image of the two slits superimposed upon the image of theclestial body when the aircraft is ying at'` constant velocity, any ch'ange in the velocity of the aircraft will cause a change in the shape of Vthe liquid wedge and hence cause the image 'of the slits to move away from the image of thecelestial body. Inasmuch as it is impossible for the Vobserver to tell when the velocity of the aircraft is constant and when it is changing, his only recourse is to take a number of readings from the instrument and to then obtain the mean thereof which, according tothe law of probabilities, should reprsent the true altitude of the celestial body.

In the present device a novel mechanism is provided for obtaining an accurate reading from the sextant. As shown in Fig. 3,' a milled handwheel 14 is secured to one end of. a shaft '16 (see Fig. l) which is journaled in a boss 'I8 formed on the sidewall of the frame l0. The. handwheel 14 is located on the outside of the yframe in close proximity to theright handgripfof the sextant (not shown) so that the wheel may be manipulated by the thumb and lingers ofthe observers right hand at the same time that the grip is being held with this hand.

'A bevel gear 80 is mounted on'the inner end of the shaft: 18 by means of a clamping collar 82, whchis secured 'tothe gear? 80. The degree of tightness with which the collar grips the shaft 16 may be adjusted by tightening or loosening a screw .84 provided in the collar. By means of this construction, a slip-clutch arrangement is provided between the handwheel and the gear; i. e., the amount of torque which can be applied to the'gearby the handwheel is limited to a denite amount after which the gear will merely slip on the shaft. l

A limit stop 86 is provided on the shaft 15 between theins'ide wall of the frame l0 and the face of the bevel gear 80 for limiting the number of revolutions through which the handwheel may be turned in either direction. This stop consists of anumberof disks or washers 88 each bearingan ear 90 which is swaged over at its outer end so'as` to overlap the ear on the next disk to the vleft as viewed in Fig. 3. The leftmost disk 92 is rigidly secured to the frame v4l0 and the rightmost disk 94 is rigidly secured to the shaft 16, while the intermediate disks are freely rotatable on'the shaft 18. By means of this arrangement, the handwheel 14 may turn through approximately as many revolutions from one limit to the other as there are free disks 88. v

Meshing with the bevel gear is a small bevel gear 96 rigidly secured to the upper end of a vertical shaft 98 by means of a Aclamping collar |00. The'lshaft 98 is journaled vatiits upper end in'a'b'racket |02 secured to the frame I0 and atits'lower end in a bushing |04 mounted in the bottomwall of the frame l0. A gear l06 is pinned tov the lower end of theY shaft 68 and meshes with a wide-faced gear |08, which is rotatably mounted on a shouldered screw H faste'ncdto' the. bottom wall of the frame. The gear |08 meshes with another wide-faced gear H2 which is rigidly secured to a vertical shaft H4 by means of a clamping collar H5. The lower endy of the shaft H4 is journaled in a bushing H81 mounted in the bottom wall of the frame I0 while the upper end of the shaft is secured by a` clamping collar. |20 to the lower end of a hub |22 of a minutes counter drum |24. A second vertical shaft |26, which is located in vertical alignment with the shaft H4' is pinned at its lower end to the hub |22. Since the minutes counter drum is rmly secured to the shaft H4 itwill be constrained to revolve at the same speed as this shaft. The registeringr mechanism ofthe sextant also includes a degrees counter drum |28 and a tens of degrees counter drum |30, which two drums are rotated by the shaft.

|26 at a reduced speed by means of suitable internal gearing (not shown). As shown in Fig. l, the minutes drum is marked off with two sen ries of graduations each extending from 0 to 60. Hence, one-half of a revolution of this drum is equal to oneI degree of altitude. The degrees Idrum is marked off with the numbers from one to ten, as is also the tens of degrees drum. Accordingly, the drum |28 is arranged to rotate through one complete revolution for every five rotations of the drum |24, while the drum |30 will turn through one revolution for every ten revolutions of the drum |28 or fifty revolutions of the drum |24. The altitude of the celestial body beingv observed may be read off of the drums |24, |28 and |30 after the observation has been completed by virtue of the counter drum viewing window |32 provided in the side wall of the frame l0.

The upper end of thel shaft |26V is firmly secured to the lower end of the worm 22 by means of a clamping collar |34. The worm is mounted for rotation within a tube |36 by means of two ball bearings |33 and |00 mounted on the inside of the tube. The tube is flexibly attached at its lower end to the seXtant frame by means of a thin washer |42, whose inner annulus is secured to the bottom of the tube |36 and whose outer annulus is clamped in the ring |44 which is secured to the frame l0. The teeth of the sector 24 are constantly maintained in close contact with the worm by means of a spring pressed plunger |46- which is supported in the frame I0 and is pressed against the outer surface of the tube |36 by the spring |48.

It will be apparent from the foregoing that manipulation of the handwheel 14 will cause the index mirror l2 to be rotated about its axis 20, this result being effected by the gear train 80, 96, |06, |i2, 22 and 2li. It will also be apparent that the position of the mirror |2, or, in other words, the altitude of the celestial body, may be read oif directly from the several counter drums of the register through the viewing window |32. Itis also to be noted that the rotation of the mirror in either direction is limited by the. limit stop 86 so as to prevent the teeth of the sector 24 from becoming disengaged lfrom the thread of the worm 22.

Coming now to the novel feature of the present invention, a means is provided in the seXtant herein shown and described for limiting the rate at which lthe mirror I2 may be rotated by the handwheel 14, this means taking the form of an escapement mechanism in theA presentl embodiment of the invention. As shown in Figs. 1 and' 3, .the wide-faced gear |08 meshes with the upper edge of the gear H2 and also with'v another'gear |50 which is freely rotatable ona vert-ical'- shaft |52. The shaft |52 is journaled at its upper end in a projection |54 extending from the bracket 35 and at its lower end in a bushing |56 inserted in the bottom wall of the frame |0. Another gear |58, similar to the gear |50, is loosely mounted on the shaft |52 directly below the gear |50 so as to mesh with the widefaced gear H2. By virtue of this arrangement, the two gears |150 and |58. will be rotated at equal speeds but in opposite directions. Two coil-grip friction clutches |60 and |62 encircle the shaft |52, the' clutch |60 having its' lower end connected to the' gear |50 and the clutch |62 having its-lower end connected to the gear |58. Both clutches |60 and |62 are wrapped in the same direction about the shaft |52 so that they will` alternate inY driv-d ing the shaft |52; i. e., when ,the gears' |00 and H2 are rotating in. one direction', the element |60 will slip on the shaft whilel the element |62 will seize the shaft and drive it, whereas, when the gears |08 and H2 are rotating in` theV opposite direction, the element |52 will slip on the shaft and the element |60 will seize the shaft and drive it. In this way, the shaft |52 will.V always be driven in the same direction regardless of the direction of motion of the handwheel 14.'.

Secured to the shaft |52 abovethe' gears |50 and |58 is the lower element |64 of a dog clutch |66. The upper element |68fof the'clutch isfreely roftatable on the shaft and is provided: with'an annular groove |10 which is engaged' by ay pair of pins |12 fastened to the arms ofv a yoke |14. This yoke is secured to one end ofa shaft 616.Y which.V is journaled in the bracket 35. The clutchelement I 68 may be moved up or down on the shaft |52- by means of a thumb lever |18 secured to the shaft |16 on the outside of thecover plate H. A rearwardly extending armE is secured to thesh'aft |16 just inside of the cover plate for the pur pose of retaining the yoke in either its-raised-or itsk lowered position. This is accomplished by means of a snap-over spring |82, which i-s secured at one.

end to a stationary stud |84 and at the other end to a stud |86 projecting from one end of a swinging link |88. freely pivoted on a shouldered screw |00 (see Fig. 3) which is threaded into a vertical extension |92. of a bracket |04 which is secured to the bottom wall of the frame l0 by means of screws |96. The stud |86 projects through a slot |98 formed in the rear end of the arm |80, thereby tending to move the arm |80, shaft |16 and yoke |14 to one or the other of two extreme positions;v

Rigi'dly secured to the upper element |68 of the dog clutch |66 is a gear 200 which meshes with` a wide-faced pinion gear 202 (see Fig. 2). Secured to the gear 202 is a larger gear 204 (see Fig. 3) whichl serves to drive a conventional escapement mechanism 206 having an escapement wheeli 20S, a lever 2|0, a balance wheel2l2, ya balance spring 2|4- and a regulating lever 2|6. The. escapement mechanism and thev gearing connecting the same to the shaft |52 is so constructed and arranged as to limit the rate of rotation of the mirror l2 to approximately 45 minutes of altitude perr minute.A In order to facilitate the initial adjustment of the instrument, the escapement mechanism may be disengaged from the gear train connecting the handwheel 14 with the mirror |2l by lifting the'v thumb lever |18.A

The other end of the link4 |88 is y The operation of the device is as follows:

When a sight is to be taken on the sun or some other celestial body, the lter disk 50 is rotated so as to bring an appropriate lter glass 54 in front of the aperture 4D. The thumb lever V58 is then lifted so as to disengage the clutch ISB and the observer, holding the instrument in both hands by means of the handgrips located on either side of the instrument, rotates the handwheel 14 until an approximate coincidence between the image of the celestial body and the image of the slits 60 is observed through the eyepiece 48. The thumb lever |18 is then pressed down so Vas to engage the clutch |66 and hence bring the escapement mechanism 266 into play. The observer then attempts to maintain the two bright lines of light formed by the slits 68 superimposed upon the image of the celestial body by rotating the handwheel 'i4 backward or forward as the need may arise. By Virtue of the operation of the escapement mechanism, the observer will be unable to rotate the index mirror rapidly enough to follow the deviation between the two images caused by acceleration effects on the iiuid so that the correction applied will always re main inside of the actual deviations. In a sense, this amounts to an integration of all of the deviations since the angle through which the mirror is rotated for any given deviation is dependent upon the length of time for which the deviation persists. In other words, the longer a deviation persists, the further the mirror will be rotated in that direction by the operator of the instrument. This procedure is followed until the oscillations of the slits settle down to an equal number of deviations on each side of the image of the celestial body per unit of time. The observer then lowers the sextant and notes the reading of the register through the window [32 and also the time. Using this data, the observers position on the earths surface may then be computed in the usual manner.

Having thus described my invention in conjunc-l tion with a preferred embodiment thereof, what I claim as new and desire to secure by Letters Patent is:

l. An observation instrument for determining the position Vof an object with respect to a predetermined reference point when relative motion of an oscillatory nature exists between the object and the reference point, said instrument -comprising an eyepiece for viewing an object, reference means providing a reference point Viewable in said eyepiece, optical means including a manipulable index member for enabling the image of the object and the image of the reference point to be simultaneously viewed in a superimposed relationship through said eyepiece, means for indicating the position of said index member, and means for limiting the speed of movement of said index member to a speed less than the speed of relative motion betweenthe object sighted and the reference point so as to enable an accurate reading therefrom to be rapidly obtained.

2. The invention as deiined in claim 1 wherein said limiting means comprises an escapement mechanism operatively connectedl to said index member.

3. In a celestial navigation instrument adapted for determining the position of an object with respect to a predetermined reference point when relative motion of an oscillatory nature exists be# tween the object and the reference point, said instrument having a rotatable index mirror, a hori` zon glass, and an artificial horizon, the combination of means for enabling an observer to simultaneously view the image of the celestial body and the image of the artificial horizon, manipulable means for rotating said index mirror so as to enable the image of the celestial body to be brought into a superimposed relationship with respect to the image of the artificial horizon, and means operatively connected with said manipulable means for limiting the speed with which said index mirror may be rotated to a speed less than the speed of relative motion between the object sighted and the reference point.

4. 'Ihe invention as defined in claim 3 wherein said limiting means comprises an escapemen mechanism.

5. In a celestial navigation instrument adapted for determining the position of an object with respect to a predetermined reference point when relative motion of an oscillatory nature exists between the object and the reference point, said instrument having a rotatable index mirror, a horizon glass, an artificial horizon, and an eyepiece for enabling an observer to simultaneously view the image of the celestial body and the image of the articial horizon, a manipulable member for rotating said index mirror so as to enable the image of the celestial body to be brought into a superimposed relationship with respect to' the image ofV`v the articial horizon as viewed through said eyepiece, an escapement mechanism engageable with said manipulatable member to limit the speed of movement of said index member to a predetermined rate of speed less than the speed of relative motion between the object sighted and the reference point, and a clutch mechanism for selectively engaging said escapement mechanism with said manipulable member so as to limit the speed with which said index mirror may be rotated.

6. The invention as defined in claim 5 wherein a registering mechanism is operatively connected with said index mirror for indicating the angular position of the same.

7. A sextant adapted for determining the position of an object with respect to a predetermined reference point when relative motion of an oscillatory nature exists between the object and the reference point, said sextant comprising an index mirror, a manipulable means for rotating said index mirror, and an escapement mechanism operatively connected with said manipulable means for limiting the speed of rotation of said index mirror to a predetermined rate of speed less than the speed of relative motion between the object sighted and the reference point.

8. The invention as defined in claim 7 wherein means is provided for selectively disconnecting said escapement mechanism from said manipulable means.

9. The invention as defined in claim 7 wherein a registering mechanism is operatively connected with said index mirror for continuously indicating the angular position of the same.

10. In a sextant adapted for determining the position of an object with respect topa predetermined reference point when relative motion of an oscillatory nature exists between the object and the reference point, said sextant havinga rotatable index mirror, an artificial horizon, and an optical system for enabling the image of the body being observed and the image of the artificial horizon to be simultaneously viewed by the user of the instrument, the combination of a handwheel, a gear train operatively connecting said handwheel with said index mirror, an es- --capement mechanism engageabie with said gear ltrain to limit thespeed of movementofsaid ingear train so as to continuously indicate the,

angular position of saidindex mirror.

12. The invention as defined-in claim wherein means is provided for causing said escapement mechanism to always be driven in the same direction regardless of the direction of rotation of said handwheel.

13. The invention as defined in claim 10 wherein means is provided between said handwheel and said gear train for limiting the amount of torque which may be applied to said gear train through said handwheel.

14. An observation device adapted for determining the position of an object with reference to a predetermined reference point when relative motion of an oscillatory nature exists between the object and the reference point, said device comprising sighting means for viewing an object, reference means providing a reference point viewable in said sighting means in relation to the object sighted, a manipulatable index member for indicating the angle of said object with respect to said reference point, and means for limiting the speed with which said index member may be operated to a speed less than the speed of relative motion between the object sighted and the reference point.

15. An observation device adapted for determining the position of an object with respec-t to a predetermined reference point when relative motion of an oscillatory nature exists between the object and the reference point, said device comprising sighting means for viewing an object, reference means providinga reference point viewable in said sighting means in relation to the object sighted, a manipulatable index member for indicating the angle of said object with respeot to said reference point, and means for limiting the speed with which said index member can be operated to a predetermined rate of speed less than the speed of relative motion between the object sighted and the reference point.

16. An observation device adapted for determining the position of an object with respect to a predetermined reference point, said device comprising sighting means for viewing an object, reference means providing a reference point viewable in said sighting means in relation to the object sighted, manipulative means for making an observation measurement, said manipulative means including a manipulatable index member for measuring the angle of said object with respect to said reference point, a register operable by said manipulative means for numerically indicating the result of the observation measurement, and means for limiting the speed with which said index member may be manipulated to a speed less than the speed of relative motion between the object sighted and the reference point.

17. The invention as defined in claim 16 wherein said limiting means includes an escapement mechanism.

18. An observation device adapted for determining the position' of'an object-with respect to a predetermined reference'point, said devicecomprising sighting means for f viewing an object, reference means providing areference point viewable inA said sighting-means .in relation to the object sighted, manipulativemeans for :making an observation measurement, said manipulative means including amanipulatable index member for 4indicating fthe angle-of .said .object with `respect to said reference -point, aniescapement mechanism cooperable `with said manipulative means for `limiting the'fspeed of-movement-ofsaid vindexmember to va predetermined 'rate ofspeed less than the `speed of relative motion between the object sighted and-the Yreference'point, and a clutch for selectively engaging said escapement mechanism with said manipulative -means to thereby limit the speed `'with which said index member may be manipulated.

19. An observation device adapted for determining the position of an object with respect to a predetermined reference point when relative motion of an oscillatory nature exists between the object and the reference point, said device comprising sighting means for viewing an objec-t, reference means providing a reference point viewable in said sighting lmeans in relation to the object sighted, manipulative means for making an observation measurement, said manipulative means including a manipulatable member, an index member for indicating the angle of said object with respect to said reference point, and a gear train operatively connecting said manipulatable member with said index member, an escapement mechanism engageable with said gear train to limit the speed of movement of said index member to a predetermined rate of speed less than the speed of relative motion between the objec-t sighted and the reference point, and a clutch for selectively engaging said escapement mechanism with said gear train to thereby limit the speed with which said index member may be operated by said manipulatable member.

20. The invention, as dened in claim 19, wherein a register is operably connected with said gear train for indicating the position of said index member.

21. An observation device adapted for determining the position of an object with reference to a predetermined reference point when relative motion of an oscillatory nature exists between -the object and the reference point, said device comprising sighting means for viewing an object, reference means providing a reference point viewable in said sighting means in relation to the object sighted, a manipulatable index member for indicating the angle of said object with respect to said reference point, and selectively operable means for limiting the speed with which said index member may loe operated to a speed less than the speed of relative motion between the object sighted and the reference point.

22. The invention as defined in claim 14 wherein said limiting means includes an escapement mechanism.

23. An observation device adapted for determining the position of an object with respect to a predetermined reference point, said device comprising sighting means for viewing an object, reference means providing a reference point viewable in said sighting means in relation to the object sighted, manipulative means ior making an observation measurement, said manipulative means including a manipulatable index member for indicating the angle of said object with respect to said reference point, means for limiting the speed of movement of saidy index member to a predetermined rate of speed less thanfthe'speed -of relative motion between4 the objects cited Yand the reference point, and a clutch for selectively engaging said limiting means with said manipulative imeans to thereby llimit thev speed with which said member may bemanipulated.

Y 24. An observation instrument for determining -the position of an object with respect to a prerelationship through said eyepiece, means for indicatingthe position of said index member, ma-

nipulatable means for rotating said index member, and means for limiting the speed of rotation of said index member to substantially 45 minutes of arc per minute.

, 25. A sextant adapted for determining the position of an object with respect to a predetermined reference point when'relative motion of an oscillatory nature exists between the object and the reference point, said sextant comprising an index mirror, a manipulable means for rotating said index mirror, and an escapement mechanism operatively connected with said manipulable means for limiting the speed of rotation of said ,index mirror'to vsubstantially forty-five minutes of arc per minute.

THOMAS L. THURLOW. 

